1. Field of the Invention
The present invention relates to valves and valve wear, particularly valve nut wear for a valve having a valve stem and a valve stem nut that moves the valve stem responsive to a rotation of the valve stem nut. More particularly, the present invention relates to a method and apparatus for detecting valve stem nut wear for any valve that has a valve stem operated by rotation of a valve stem nut and wherein this apparatus is not an as-manufactured part of the valve to be inspected.
2. General Background of the Invention
A motor operated valve or “MOV” apparatus can employ a rising stem. This rising stem is operated by a rotating stem nut. One of the problems that plagues users of such valves (e.g. chemical plants, nuclear plants and other such facilities) is the failure of the stem nut after it has been in service for a long time.
A motor operated valve stem nut is typically made of a softer material (such as a bronze alloyed material) than the material used to make the stem (usually steel). Over a period of time, the stem nut threads will wear when opening and closing the valve. The wear can be influenced by several factors. These factors can include the stem nut material, normal operating loads, and maximum loads as examples. The wear can also be influenced by the number of stem nut rotations that occur during the valve stroke. Another factor that influences wear is the number and frequency of valve strokes. Yet other features that contribute to wear include stem nut manufacture and threaded length, stem-to-stem-nut fit, valve stem thread nut condition, and stem lubricant. Other factors include the lubrication method and frequency as well as environmental conditions for the lubricant.
Since the valve stem is typically made of a harder material such as stainless steel or carbon steel, it is typical and expected that all thread wear will occur on the valve stem nut threads and not on the valve stem. This is preferential to having wear occur on the valve stem (as the stem nut is typically much easier to change out than the valve stem).
On rising stem valves, a stem nut transfers rotational motion (torque) of a motor operated or manual valve to axial stem movement or “thrust”. Stem nut failure will prevent valve operation and may cause valve position indication to be incorrectly displayed in a control room. If the motor operated valve is electrically interlocked, its failure can also interfere with the operation of other plant equipment resulting in a potentially costly or catastrophic failure event.
Unfortunately, many facilities, plants, chemical refineries and the like do not take a proactive approach to maintaining these valve stem nuts. One reason that these facilities do not take a proactive approach is that it is difficult to predict failure of any particular valve. In some cases, a failure may take many years to occur.
Another problem with a proactive approach to maintaining valve stem nuts is that direct inspection of the stem nut threads is intrusive and requires taking the valve out of service to remove and inspect.
On smaller diameter stems, measuring the stem nut thread wear on a removed stem nut can be very difficult. There is limited space inside the stem nut to get a direct measurement with dial calipers.
Excessive stem nut thread wear represents a potential common cause failure mode that could impact all rising stem valves. The unexpected failure of valve stem nuts, and resulting consequence emphasizes the importance of improving conditions, monitoring maintenance practice activities, and identifying, quantifying and minimizing stem nut thread wear. Periodic stem nut removal and inspection or measuring thread backlash may be beneficial to prevent failure. It is important for any maintenance program to detail proper stem cleaning and lubrication procedures.
The valves to be tested are not always gate valves, but any valve using a threaded stem and nut.
An assumption is that the stem is cut properly (which is not always the case)—thus, as part of the method, it is preferable to also measure stem threads to make sure that the assumption is correct.
At some facilities, the stems tend to corrode instead of wearing.
The tool of the present invention was validated in lab tests, where it appears that the tool can predict wear with 3-11% accuracy.
Typically, stems are made of hardened steel, but some facilities use 17-4 PH stainless steel for replacement stems.
The stem nuts can be aluminum bronze (softer), but some are manganese bronze (harder).
Some stem nuts may be as hard as the stems (that is not desirable, as the stems may wear as fast as the nuts, and they are more difficult to replace).
The stems at some facilities are 29 degree Acme thread with double leads.
The following table lists possibly relevant patents and applications, the order of listing being chronological, the order of listing otherwise having no significance.
TABLEISSUE/PUB. DATEPATENT/PUB. NO.TITLE(MM-DD-YYYY)3,721,133Anti-backlash Screw JackMar. 20, 19733,857,547Automatically Compensating JackDec. 31, 1974ConstructionJP 04296268Valve Actuator Provided with AutomaticOct. 20, 1992Detecting Means for Stem Nut Wear5,224,512Valve stem driving apparatusJul. 6, 19935,487,302Method and system for measuring gate valveJan. 30, 1996clearances and seating force5,620,166Stem and nut with bearing for use in a motorApr. 15, 1997operated valve2002/0095986Method and apparatus for diagnosingJul. 25, 2002abnormality and estimating degradation invalve apparatus6,799,928Device for detecting wear of a nut in aOct. 5, 2004screw-and-nut assembly7,549,440Valve position indicator for buried valvesJun. 23, 2009